In general, before their assembly, directly after the production of the conductor track structures, printed circuit boards are provided with a protective layer on the exposed copper surfaces and in the copper-plated drill holes, whose purpose is to guarantee that all soldering points that are to be formed during assembly satisfy all requirements both electrically and mechanically. The protective layers thus serve to safeguard the solderability and are often described as “final solderable surfaces”.
Solderable surfaces are currently normally produced in the industry from liquid solder (tin-lead or silver-tin solders, so-called “hot air tinning”) or chemically deposited nickel-gold, silver, palladium or tin layers. Such metallic finishes are designed for a storage time of up to one year, while purely organic finishes (so-called “OSP”), which are obtained by the treatment of the printed circuit board surface with complexing agents for copper, cannot usually be stored for longer than 3 to 6 months, if the solderability is to be maintained. As complexing agents, for example imidazoles, benzimidazoles, benzotriazoles, thiourea and imidazole-2-thione are used.
Metallic coatings are in general very suitable for printed circuit boards, however they also display a number of disadvantages. Coatings with gold are expensive not only on account of the high gold price, but in addition require special processes for the application of the gold layer. For example, gold cannot be applied chemically in so-called horizontal plants but only in vertical plants, which additionally results in high process costs.
The application of silver is poorly reproducible, and the necessary plants are difficult to adjust.
In recent years, therefore, there has been increasing interest in chemical tinning, not least because of enormous improvements in properties which it was possible to achieve on the basis of tin deposition with the aid of the organic metal polyaniline (“ORMECON CSN process” of Ormecon GmbH, Ammersbek), but also through intensified endeavours by the firm Atotech (“Stannatech” process). Tin surfaces are considerably more economical than for example coating with Ni/Au.
From EP 0 807 190 B1, a process for the production of metallized materials is known, wherein the material to be metallized is first coated with an intrinsically conductive polymer, the intrinsically conductive polymer is then activated by reduction and finally the metal is applied in a non-electrochemical manner, in that the coated material is brought into contact with a solution of ions of the metal. The process is particularly suitable for the deposition of tin onto copper but also for the metallization of plastic surfaces.
A disadvantage of tin surfaces can be seen in the fact that these more or less strongly tend to form, in the course of the storage time, so-called “whiskers”, i.e. needle-shaped tin crystals, which can reach several micrometres in length and which can severely impair the function of a printed circuit board. Such incidents some years ago finally resulted in tin surfaces only finding reluctant acceptance for certain applications such as flexible printed circuit boards, “tape automated bonding” (TAB) processes and “dupon film” (COF) processes, and in certain regions, such as Japan.
It has not so far been possible completely to elucidate the cause of the whisker formation, however the general view is that mechanical stresses which are transmitted through the printed circuit board into the copper and tin layer, or arise in the tin-coated Cu layer, cause the needle formation in the course of a tension-induced crystallization.
After a storage time of 7 weeks at room temperature, conventional tin surfaces display whiskers with a length of
30 μm to more than 100 μm. The whisker formation can be detected particularly well microscopically in drill holes with a diameter of 0.6 to 1.2 mm diameter. Whiskers also arise at other points, but their assessment with the microscope is easier in drill holes. After 7 weeks, tin surfaces created by the ORMECON CSN process display whiskers about 10 to 20 μm in size.
U.S. Pat. No. 6,361,823 discloses a process with which whisker formation is allegedly completely prevented. In this process, printed circuit boards are first provided with a layer of essentially pure tin, onto which an alloy layer of at least two metals is applied. The preferred alloying metals are tin and silver. Practical tests have however shown that whisker formation could admittedly be reduced, but not reproducibly and not to the desired extent.
The firm Unicron GmbH, Kirchheimbolanden, developed and for a time marketed a process wherein first a silver layer is deposited onto the exposed copper surfaces, then the tin layer is deposited by the UNICRON process (G 2). However this process brought hardly any improvement, on the one hand since with too little silver deposition no effect whatever could be achieved, and on the other hand with too great a silver layer thickness (beyond as little as ca. 40 nm layer thickness), tin was no longer deposited. Ultimately, the desired result could not be achieved with this process.
Thus, as before, there was still a need for the provision of protective coatings for printed circuit boards, which display no tendency or only a slight tendency to whisker formation.